In the string of oxymora in which she accuses Romeo of being untrustworthy, we can tell by some of her diction choices that Juliet is most troubled by the fact that Romeo has just killed someone, let alone her own cousin. One thing to remember about Juliet is that she deeply values her religion, and murder is most definitely against her religion. Therefore, now that Romeo has killed her cousin, she is seeing him as one who has trespassed against her religion, making her deeply distrust him. Several images in her speech filled with oxymora shows us that she sees him as a dangerous murderer and a sinner.
The first image depicting Romeo as a murderer and sinner is "serpent heart" (III.ii.76). In Genesis, the adversary in the form of a serpent tempted Eve to eat the fruit from the forbidden tree of knowledge of good and evil. Therefore, now, the serpent has become a religious symbol for all things evil, destructive, and sinful. Hence this one little image exposes Juliet's mind to show us how much of an evil sinner Juliet now thinks Romeo to be. Not only is a serpent a symbol of sin, serpents are often poisonous and known to be deadly. Therefore, the serpent image also serves to characterize Juliet's belief that Romeo is a murderer. The same case can be made for the dragon image in the line, "Did ever dragon keep so fair a cave?" (77). Western Christian culture recognizes dragons to symbolize destruction and evil, just like the serpent. Therefore, again with this image, Juliet is showing she thinks Romeo is a destructive, murdering sinner.
As for advice, we really can't give her any advice greater than her own resolution later in this same scene. She later realizes it is wrong of her to think illy and to distrust her husband. She also realizes that Romeo must have killed Tybalt out of self-defense.